In most machines and apparatuses which use oil in their operation or for lubrication, it is important to ensure that the oil level does not drop below a predetermined level. Due to the normal operating characteristics of many of these machines and devices, it is common for there to be a temporary drop in the level of oil within a sump although the oil level within the whole machine or apparatus remains at an acceptable level. In such instances, it is not appropriate to provide an alarm or other warning of the drop in oil level. Such an alarm would give the false impression that the overall oil level is inadequate and result in further oil being added which in itself may have adverse effects.
To avoid the above situation, it is known to provide oil level switches or monitoring devices which have in-built electronic time delays. These devices will not give an indication of a low oil level unless the level remains low for a predetermined period of time. However, the electronic implementation of time delays has several inherent problems. One difficulty, relates to the interfacing with the electrical supply associated with the machine or apparatus. For example, in a refrigeration system, the supply voltage may be between 20 volts to 240 volts AC. Also, the frequency of the supply can vary from system to system and country to country. For example, the mains frequency in Australia is 50 hertz whereas in the USA it is 60 hertz. In addition, while electronic circuits can relatively easily provide short time delays using simple capacitor circuits, to produce long time delays digital circuits and/or software control is required. This increases the complexity and costs of the time delay monitoring device.